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From the "Form Input View Options" section of the SOAtest User's Guide, which applies to Form JSON view as well:

To have values stored in a data source dictate whether a value is sent for an optional element:
1. Right-click the element’s tree node.
2. Choose Use Data Source> Exclude Element with Empty String.
3. In the tool configuration panel, ensure that the appropriate data source column is selected under Use data source: Exclude with empty string.